The difference is that understanding what a gear is is not essential to the functioning of a car at all. You can completely remove it without loss for that part

The difference is that understanding what a gear is is not essential to the functioning of a car at all. You can completely remove it without loss for that particular user nor for the community of car users. Whereas not knowing that a URL is a piece of text which specifies a particular content on a domain and can thus be copied and used for linking is a loss both for the user and for the community. You'll have to reinvent the principle of linking in another form to avoid the loss (e.g. every web and local app would need special GUI functionality to use instead, and the need for specification what to link would not be completely removable, unlike the gear).